Research Article A Three-Dimensional Porous Conducting...

Research ArticleA Three-Dimensional Porous Conducting PolymerComposite with Ultralow Density and Highly SensitivePressure Sensing Properties

Jin-Dong Su,1 Xian-Sheng Jia,2 Jin-Tao Li,2 Tao Lou,3 Xu Yan,2,4 Jia-Lin Sun,1

Jun-Hong Chen,1 and Yun-Ze Long2,4

1School of Material Science & Engineering, University of Science & Technology Beijing, Beijing 100083, China2Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao 266071, China3College of Chemical Science & Engineering, Qingdao University, Qingdao 266071, China4Industrial Research Institute of Nonwovens & Technical Textiles, Qingdao University, Qingdao 266071, China

Correspondence should be addressed to Jun-Hong Chen; [email protected] and Yun-Ze Long; [email protected]

Received 24 July 2016; Accepted 28 November 2016

Academic Editor: Stefano Bellucci

Copyright © 2016 Jin-Dong Su et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

An ultralight conducting polyaniline/SiC/polyacrylonitrile (PANI/SiC/PAN) composite was fabricated by in situ polymerizationof aniline monomer on the surface of fibers in SiC/PAN aerogel. The SiC/PAN aerogel was obtained by electrospinning, freeze-drying, and heat treatment. The ingredient, morphology, structure, and electrical properties of the aerogel before and after in situpolymerization were investigated by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanningelectron microscope (SEM), and voltage-current characteristic measurement. The thermostability of PANI/SiC/PAN compositewas investigated by thermogravimetric analysis (TGA) and electrical resistance measured at different temperatures. The densityof the PANI/SiC/PAN composite was approximately 0.211 g cm−3, the porosity was 76.44%, and the conductivity was 0.013 Sm−1.The pressure sensing properties were evaluated at room temperature. The electrical resistance of as-prepared sample decreasedgradually with the increase of pressure. Furthermore, the pressure sensing process was reversible and the response time was short(about 1 s). This composite may have application in pressure sensor field.

1. Introduction

Various sensors were widely used in all walks of life, andthe fabrication of high-performance sensor has drawn exten-sive attention. Using nanomaterials (nanoparticles [1, 2],nanofibers [3, 4], nanotubes [5, 6], and aerogels [7, 8]) espe-cially nanofibers or nanofibrous membrane as the element ofsensors is effective means to perform high-performance,pint-sized, and lightweight sensors because the low densityand high specific surface area can reduce the weight and imp-rove performance of sensor.

Since aerogel was first discovered by Kistler [9] in the1930s, many efforts have been paid to the developmentof aerogels because of many kinds of excellent properties.Aerogel, as the lowest density solid material at present, has

an extreme porosity of approximately 90%–99%. The partic-ular three-dimensional (3D) porous structure made it havegood performance in thermal insulation [10, 11], absorptionof oil [12, 13], catalyst support [14, 15], drug carrier [16],and lithium-ion batteries [17, 18]. Aerogels as gas sensorelement have good performance and because of that thelarge mesopores can provide favorable diffusion for free mol-ecules to move reversibly from the external environment tothe pore interior [19]. Nowadays, aerogels have been used inhumidity sensing [19, 20], oxygen sensing [21], nitrogen dio-xide sensing [22] and other sensors. However, most aerogelsneed to composite conductive materials to enhance conduc-tivity. Polyaniline (PANI), as a conductive polymeric mate-rial, has low density, good electrical conductivity, and plas-ticity. Combining PANI with other materials can obtain good

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2016, Article ID 5164012, 8 pageshttp://dx.doi.org/10.1155/2016/5164012

2 Journal of Nanomaterials

conducting composite materials. There are many reportsabout in situ polymerization of PANI on the surface of org-anic poly(methyl methacrylate) (PMMA) nanofibers [23],poly(vinylidene fluoride) (PVDF) nanofibers [24], and soon to obtain composite materials which have good sensingproperty. However there are little reports about in situpolymerization of PANI on the surface of nanofibers infibrous aerogels.

Here, we used the SiC/polyacrylonitrile (PAN) compo-site nanofiber aerogels as support to fabricate a porous con-ducting material with ultralow density by the in situ poly-merization of PANI on the surface of fibers in aerogels. Themass increase is 400% after in situ polymerization.The PANIdeposited on the fibers in aerogel and became the majority ofPANI/SiC/PAN composite, so the composite has high elec-trical conductivity.The conductivity of PANI/SiC/PAN com-posite is about 0.013 Sm−1. The 3D porous PANI/SiC/PANcomposite is of low density and high porosity. Its density is0.211 g cm−3 and its porosity is 76.44%. The sample has highperformance in pressure sensing. The response of pressure isreversible and the response time is short. Under the constantvoltage, the current can increase 500% after pressing for 1 sand increase 750% after pressing for 30 s.

2. Experimental

2.1. Fabrication of Aerogels. Thepolyacrylonitrile (PAN) nan-ofibers were prepared by electrospinning. The electrospinn-ing precursor solution contained 10wt% PAN (𝑀𝑤 ∼5,000,000) and 90wt% DMF (AR). 0.75 wt% PAN nanofibermembrane, 0.25 wt% SiC nanowhiskers (diameter of 200–500 nm), 20wt% tert-butyl alcohol, and 79wt% deionizedwater were put in beakers, and then they were dispersedby high-speed dispersion homogenizer (A-25, China) with15,000 rpm for 15min. Then the dispersed mixture waspoured into weighing bottles in a plastic box. Liquid nitrogenwas poured into plastic box to freeze the solution quickly.Then the weighing bottles with freeze casting PAN/SiC weretransferred to freeze-dryer (FD-1-50, China) under 50 Pa and−50∘C for two days. So the SiC/PAN aerogels were prepared.The samples have no elasticity, so the samples were put in airdry oven for 300∘C heatingmodified to obtain good elasticityand morphological stability.

2.2. In Situ Polymerization of PANI on Fibers in SiC/PANAerogel. The PANI/SiC/PAN aerogels were fabricated by thein situ polymerization of PANI on the surface of PAN or SiCfibers. In the progress of in situ polymerization, ammoniumpersulfate (APS) was used as oxidant and 5-sulfosalicylic aciddihydrate (SSA) was used as catalytic agent, and the anilinewas themonomer to form polymer by in situ polymerization.The specific procedures were as follows. Firstly, two solutionswere prepared: 4.66 g APS was dissolved in 100mL deionizedwater marked as solution A and 7.98 g SSA and 2.26 g anilinewere dissolved in 100mL deionized water marked as solutionB. Secondly, SiC/PAN aerogels were put into solution B, andthen solution A was instilled into solution B with ultrasonicconcussion by ultrasonic vibrating machine (KQ2200DE).Thirdly, the mixed solution was put in freezer at 3∘C for

Electrospinning PAN nanofibers

Homogeneous dispersion of PAN nanofibers and SiC whiskers

In situ polymerization of PANI

Preparation of aerogels by freeze-drying method

Heating modification

Figure 1: The preparation flow of PANI/SiC/PAN composite.

24 h. Fourthly, the aerogels were taken out and washed bydeionized water. At last, the PANI/SiC/PAN porous compos-ites were dried by natural drying. The fabrication process isshown in Figure 1.

2.3. Pressure Sensing Test. As shown in Figure 2, pressure sen-sing property was tested by simple homemade device. Twocopper wires were fixed on the sides of the sample aselectrodes by silver colloids.Then two glass slides were affixedon the sides of the sample to stable the electrodes and sufferthe pressure. Two electrodes were connected to Keithley 6487high resistance meter system to test the change of electricalconductivity. In the process of testing, a constant voltage of5Vwas supplied, so the changes of instantaneous current canreflect the changes of electrical conductivity of sample underpressures.

2.4. Characterization. The microstructure of aerogel beforeand after in situ polymerization was measured by a scan-ning electron microscope (SEM, Nova NanoSEM 450). Thesample before polymerization was sputtered with gold andtension of 10 kV was applied. Tension of 20 kV was appliedto the sample after polymerization. The thermal stabilitywas detected by thermogravimetric analysis (TGA, Mettler-ToledoTGA/DSC).The analysis was performed at 10∘Cmin−1from 25∘C to 500∘C under air atmosphere. The thermogravi-metric analysis testing under air atmosphere is because wewant to know the temperature which can be practically used.And the resistances under temperature from 25∘C to 250∘Cwere tested by the High Precision Heating Stage (SET3625,ShenZhen, China) and multimeter; the resistance increasedacutely under 225∘C, so the testing was only up to 250∘Cnot 500∘C. The phases were identified by X-ray diffraction(XRD, RIGADU D/MAX-2500) in the angular 10−90∘ witha scan speed of 4∘min−1. The chemical compositions wereanalyzed by Fourier transform infrared spectroscopy (FT-IR) using a Thermo Scientific Nicolet iN10 spectrometer andtransmittance datawere processed for thewave number range700–4000 cm−1. The conductivities of the sample before and

Journal of Nanomaterials 3

Electrode

Press

Sample

Glass slide

Glass slide Electrode

Keithley 6487 high resistance meter system

Figure 2: Diagrammatic drawing of pressure testing equipment.

Table 1: Morphology, conductivity, and porosity changes of the aerogel after in situ polymerization.

Diameters (cm) Height (cm) Weight (g) Conductivity (Sm−1) Porosity (%)Before polymerization 2 1.1 0.0323 2.9 × 10−8 99.24After polymerization 1.2 0.6 0.1623 1.3 × 10−2 76.44

after in situ polymerization were calculated from voltage-current curves measured by a Keithley 6487 high resistancemeter system.

3. Results and Discussion

The conducting aerogel was prepared by the in situ poly-merization of PANI on the surface of fibers in the aerogels.The pores (diameter of about 1 𝜇m) in as-prepared PAN/SiCaerogel are bigger than pores in common aerogels, so thePANI is more easily polymerized in the inside of aerogel andthe coating layer of PANI can be thicker. We can see that thecolor of the sample was translated to bottle green as shown inFigure 3, and it is due to the PANI being bottle green. In fact,in the process of polymerization of PANI, the solution andthe sample changed color gradually. And the color of samplewould not change back after cleaning by deionized water. Itindicated that the PANI was in situ polymerization on thesurface of fibers in aerogel firmly and stably. It can be seenfrom Figure 3(b) that there is shrinkage after polymerizationcompared with original aerogel. It should be noted that theshrinkage of the sample happened in the process of drying,and there are almost no changes of shape and volume in theprocess of polymerization and washing. The shrinkage maybe due to the surface tension of water in the process of drying.The weight of the sample was increased from 0.0323 g to0.1623 g; the increase of relative mass is 400%. So the densityafter polymerization was increased (about 0.211 g cm−3). Theporosity of aerogel was calculated by the formula 𝑃 = (1 −𝜌𝑏/𝜌𝑠) × 100%. Here, 𝜌𝑏 is the bulk density, 𝜌𝑠 is the skeletaldensity, and 𝑃 is the porosity.The porosity of PANI/SiC/PANcomposite was 76.44%.

The I-V curves of the sample before and after in situ poly-merization were shown in Figure 4. The resistance of sampleafter polymerization was increased 2 × 107 times comparedwith the original sample. The specific morphology changeof aerogel after polymerization was shown in Table 1. The

conductivity changes were not equal to resistance changebecause of the shrinkage of sample. The specific conductivitywas changed from 2.9 × 10−8 Sm−1 to 1.3 × 10−2 Sm−1 cal-culated by the formula 𝐺 = 𝐼/𝜌 = 𝑆/𝑅𝐿; here 𝐺 is the con-ductivity, 𝜌 is the specific resistance, 𝑆 is the area of surface,𝑅 is the resistance, and 𝐿 is the length.

The FI-IR spectra of the sample before and after in situpolymerization are shown in Figure 5(a). The measurementrange was 700–4000 cm−1. The SiC/PAN aerogel spectra(curve (A)) exhibited the absorption peaks of a stretchingvibration at peaks at 2933 cm−1 (CH stretching in CH andCH2 groups), 2242 cm

−1 (C≡N), 1451 cm−1 (CH blending),and 1119 cm−1 (ether band) [25, 26], and those four peaksthat come from PAN nanofibers cannot be observed in thespectrum of PANI/SiC/PAN composites which may due tothe PAN nanofibers being clad by PANI. Absorption peaksat 802 cm−1 and 917 cm−1 are ascribed to the transversaloptic (TO) mode and longitudinal optical (LO) vibrationof the Si-C stretching vibration [27]. Those two peaks wereweakened after polymerization. The characteristic peaks at1657 cm−1 and 1567 cm−1 are attributed to the stretchingvibration of quinoid and benzenoid rings on PANImolecularchain, respectively [28]. The peak observed at 1080 cm−1is designated to vibration mode of N=Q=N which is anelectronic band (Q refers to the quinonic type rings) [29].The C-H vibration of aromatic ring at 3059 cm−1 and C-N offrom aromatic amines at 1297 cm−1 [30] were also observed incurve (B). All those peaks proved the in situ polymerizationof PANI on the surface of fibers in aerogels.

The XRD patterns of the sample before and after in situpolymerization are shown in Figure 5(b). The peak at 2𝜃values of 16-17∘ is related to PAN [31], which can both beobserved from curve (A) and curve (B). The peaks at 35.9,41.6, 60.1, and 71.9 corresponded to (111), (200), (220), and(311) crystal planes of 3C-SiC [32], which can also be bothobserved from curve (A) and curve (B).The crystalline peaksappear at 2𝜃 = 15.2, 20.7, and 25.4, corresponding to (011),


(a) (b)

(c) (d)

Figure 3: Photographs of (a) PAN/SiC aerogel and (b) PANI/SiC/PAN composite and SEM images of (c) PAN/SiC aerogel and (d) porousPANI/SiC/PAN composite.

(b)

(a)

(a)

0 2 4 6 8 10 12 14 16−2

U (V)

15.017.512.510.07.52.5 5.00.0U (V)

0.00

0.250

0.500

0.750

1.00

1.25

1.50

I(A

)

×10−10

×10−3

−0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

I(A

)

Figure 4: The I-V curve of sample before (a) and after in situ polymerization (b). The detail of curve (a) was shown in top left corner.


1297

3059

1080

1657

1567

917

802

14511119

2242

(B)Tr

ansm

ittan

ce (a

.u.)

(A)2933

1000 1500 2000 2500 3000 3500 4000500Wavenumber (cm−1)

(a)

71.9

60.1

41.6

35.9

25.4

(B)

Inte

nsity

(a.u

.)

(A)

15.2

20.7

20 30 40 50 60 70 80 90102𝜃 (degree)

(b)

Figure 5: (a) FT-IR spectra and (b) XRD patterns of (A) SiC/PAN aerogel and (B) PANI/SiC/PAN composite.

0

20

40

60

80

100

120

Wei

ght (

%)

100 200 300 400 5000Temperature (

∘C)

(a)

25 50 75 100 125 150 175 200 225 250 2750Temperature (

∘C)

0

10

20

30

40

50

60R/R

0

(b)

Figure 6: (a) TG curve and (b) the resistance variation with increasing temperature of PANI/SiC/PAN composite.

(020), and (200) crystal planes of PANI, respectively [33],which can observed from curve (B). All the peaks illustratethe existence of SiC, PAN, and PANI.

The thermogravimetric (TG) curve of PANI/SiC/PAN isshown in Figure 6(a). The weight loss from room tem-perature to 78∘C is due to the presence of moisture. ThePANI/SiC/PAN composite can absorb a large number ofwater molecules because of the high porosity. The obviousweight loss from 234∘C to 500∘C is attributed to the degra-dation polymer backbone of PANI [34] and the pyrolysis ofPAN [35].

Figure 6(b) shows the resistance variationwith increasingtemperature from 25∘C to 250∘C of PANI/SiC/PAN com-posite. The increase of resistance from 50∘C to 100∘C maybe due to the water evaporation because water can increasethe electrical conductivity of polyaniline through an increasein the interchain electron transfer and/or by increasing themobility of dopant ions [36–38]. The increase of resistance

beyond 100∘C is due to the pyrolysis of PANI. The pyrolysisof PANI is drastic after 234∘C as shown in Figure 6(a) and theincrease of resistance is also drastic from 225∘C to 250∘C.

To test the pressure sensing property, the resistances of thesample under different pressures at 10−3 bar levels were tested.As shown in Figure 7, the imposed pressures were from 0.001to 0.010 bar. The electrical resistivity declined gradually withthe increase of pressure. And the decrease of resistance wasabout 18% and even the pressure was only 0.002 bar. So thesample can detect the light contact pressure at 10−3 bar levels.The change of electrical resistance was nonlinear may be dueto the fact that the closing of pore was not completely inproportion to the pressure.

The reversible pressure sensing property of sample wasalso tested. A constant voltage of 5V was applied on the sam-ple, so the electric current reflects the change of resistance.The test period was 60 s (30 s for pressure and 30 s for relax).As shown in Figure 8(a), when the sample was pressed by the


2 4 6 8 100

Pressure (10−3 bar)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Nor

mal

ized

resis

tanc

e

Figure 7: The resistances under different pressures at 10−3 bar levels.

I/I 0

T (s)0

0

2

4

6

8

10

12

100 200 300 400 500

(a)

I/I 0

T (s)0 10 20 30 40 50 60 70 80

1.0

1.2

1.4

1.6

(b)

Figure 8: The reversible response curves (a) and the instantaneous response curves (b) of electric current to the pressure.

pressure of 0.02 bar, the current increased clearly.The currentrise quickly at first; the increase of current was up to about500% when the sample was pressed for 1 s. Then the currentrise rate was slowed down.The increase was up to 750%whenthe samplewas pressed for 30 s.When the samplewas relaxed,the resistance was regained. So the response to pressure wasfast and reversible.

The instantaneous response of pressure was also tested.The means is similar to the reversible response testing; aconstant voltage of 5V was supplied on the sides of sample.But in order to obtain quick pressure and recovery, we useindex finger to press sample in a very short time. Figure 8(b)shows the response curve of instantaneous pressure. Theresponse and recovery time is quite short, and the responseof current is clear (increased by about 50%). There is a littleincrease (about 10%) of current after pressing and recovercompared with the original current, and it may be due to theunrecoverable deformation in the process of testing.

Figure 9 shows the diagrammatic drawing of poreschanging in the process of pressing and relaxing. After in

situ polymerization the fibers were packaged by PANI. Thefiber became thicker and the pores became tiny. The electriccurrent was conducted from one fiber to another in thesample. Under the effect of pressure, the sample deformationoccurred. A portion of pores was closed when the samplewas pressed while they were open when the pressure wascanceled. The closure of pores can connect conducting fibersso the electrical conductivity of sample increased with thepressure. The shape changes become bigger and the closedpores become more with the increase of pressure, so theelectrical conductivity becomes higher with the increase ofpressure.

4. Conclusion

Here, a 3D porous conducting material was prepared by asimple and efficient method—in situ polymerization of PANIon the surface of fibers in the aerogels. After polymerizationof PANI, the weight was increased by 400%. And its


(a)

Press

(b) (c)

Figure 9: Diagrammatic drawing of changing process of pores when pressed and relaxed.

density was still low (about 0.211 g cm−3). The porosity ofPANI/SiC/PAN composite is 76.44%, and the conductivity ofit is 0.013 Sm−1. The sample has good pressure sensing pro-perty because of its porousness and good conductivity. Thepressure sensing property was tested by homemade simpledevice. The resistance of sample was reduced gradually afterthe increase of pressure. The response is reversible and theresponse time is short, so the composite can detect even ins-tantaneous or tiny pressure. The 3D porous conductingPANI/SiC/PAN composite material may be used in pressuresensors.

Competing Interests

The authors declare that they have no competing interests.

Authors’ Contributions

Jin-Dong Su andXian-Sheng Jia contributed to thiswork equ-ally.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (51673103, 51373082, and 51572019),the Taishan Scholars Programme of Shandong Province,China (ts20120528), Innovative Research Team in University(IRT1207), and the Postdoctoral Scientific Research Founda-tion of Qingdao.

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